It has been well established that fibrous constructs, as in the cellulose-based cotton gauzes, can aid the process of hemostasis through interrupting the blood flow and subsequent coagulation. Positively charged cellulosic fibers, such as those based on chitosan, were later recognized as effective hemostatic constructs. More recent understanding of the relevance of hemostasis to key biological events associated with wound healing, tissue ingrowth about prosthetic devices, and tissue engineering justified the need to explore means to modulating the hemostatic process to meet the specific requirements at different biological sites for optimum hemostasis and, hence, optimum functional performance at said sites. And this invention deals with mechanical, physicochemical, biochemical and pharmacological means to modulate the hemostatic process to meet the needs of specific individual biological events. To explore these means, this invention relies on (1) electrostatic spinning technology to produce nanometer and micrometer diameter fibers with exceptionally high surface area for maximized effect on blood flow to initiate the clotting cascade through contact activation; (2) producing inherently compliant and elastic components of the fibrous construct through the use of segmented crystalline copolymers having triaxial or polyaxial chain configuration (i.e., three or many chain branches extending outward from a central atom); (3) controlling the composition of electrostatically spun fiber precursors to provide constructs with controlled solubility and biodegradability and hence, modulated short- and long-term retention of mass and biologically relevant properties; (4) controlling the surface charge of the electrostatically spun fibers to physicochemically modulate the hemostasis at will; and (5) incorporating judiciously bioactive agents to prevent infection, pain, and/or promote desirable biological events.
The topic of hemostasis and commercially available hemostatic sealants and their mechanisms of action have been treated in a recent review [J. Cardiovascular Surgery, 18, 486 (2003)]. It was noted in this review that application of direct pressure at a bleeding site frequently provides either complete control of bleeding or adequate control to enable more definitive measures to be taken. However, this method of controlling bleeding may not be sufficient when the source of bleeding is hard to identify, as may be the case for diffuse venous bleeding, or when an inherent coagulopathy is present. Intraoperative coagulopathy, which can be induced by a variety of factors including hemodilution and hypothermia, may be treated with active hemostatic agents. Hemostasis is also compromised due to the presence of antiplatelet and anticoagulation agents, especially in patients undergoing cardiac or vascular surgery, as well as from changes associated with cardiopulmonary bypass. In such cases, topical hemostatic agents become useful adjuncts to the conventional methods of achieving hemostasis. A comprehensive list of currently available topical hemostatic agents, together with their key attributes, is presented in Table I.
TABLE IComparison of Topical Hemostatic AgentsCommercialMaterialMechanism of ActionWhat It Needs to WorkCelluloseSurgicelCellulose fibers initiateFunctional clottingOxycelclotting cascade throughcascade and all clottingcontact activationfactorsGelatinGelfoamInitiation of clottingFunctional clottingSpongecascade through contactcascade and all clottingPowderactivationfactorsFilmSurgifoamCollagenAviteneContact activation andFunctional clottingFlourpromotion of plateletcascade and all clottingSheetaggregation to initiatefactorsthe clotting cascadeActifoamContact activation andFunctional clottingSpongepromotion of plateletcascade and all clottingInstataggregation to initiatefactors.(flour)the clotting cascadeHelistat/HeliteneFlosealGelation granulesCirculating fibrinogenMatrixrestrict the flow ofHemostaticblood, provide aSealantphysical matrixaround which a clotcan form, and deliverand maintain thrombinto the tissue surfaceThrombinThrombinInteracts with fibrinogenCirculating fibrinogenJMIin the patient's blood toand a means of deliveryform a fibrin clot(customarily used withGelfoam sponges orpowder) for use onactive bleedingFibrin GlueTisseelMixes fibrinogen,Fibrin glue must beHemaseelthrombin and factor XIIIwarmed prior to useFibrin Gluedispensed from a double(20-to-40 minute process)Variantsbarrel syringe toCoStasis/Dynastat requiresCostasisgenerate a clot. Fibrinpatient's blood to beDynastatglue also includesdrawn and centrifugedaprotinin (bovine-Can only apply to a dry,derived) to preventstationary tissuefibrinolysis. CoStasis/surfaceDynastat include collagen,but not aprotininAldehydeGluesBioglueGlutaraldehyde andDry thoracic aorticalbumin cross-linked withtissueproteins in tissueTissue that canforming a strongwithstand exogenousadhesivecross-linking
The term hemostatic agent (or material) has been defined (U.S. Pat. No. 6,706,051) as any agent or material that is capable of arresting, stemming, or preventing bleeding by means other than inducing tissue growth alone. In other words, something other than tissue growth is at least partially responsible for retarding or preventing bleeding. Preferably, the agent or material will be one that enhances blot clot formation. It will, of course, be appreciated that the agent or material may have the beneficial property of inducing tissue growth in addition to retarding or preventing bleeding. Examples of preferred hemostatic agents which enhance blood coagulation include carboxymethylcellulose (CMC), oxidized cellulose, calcium alginate, gelatin, or collagen, oxidized cellulose, such as Tabotamp™, is another example of a hemostatic agent. Falling under the definition of “hemostatic agent” is the cellulose-based cotton gauze, the first and gold standard for all-time surgeons. The early successful application of the cotton gauze as a hemostatic construct led several investigators to associate the fibrous structure of the gauze with the interruption of blood flow by initiating the clotting cascade through contact activation. This, in turn, provided the incentive to examine other fibrous constructs made primarily from natural materials, such as collagen, chitosan, and alginate, which led subsequently to the discovery of surface charge contribution to hemostasis. With the development of synthetic absorbable/biodegradable fibers, several investigators directed their attention to fibrous synthetic polyester as transient alternatives or additives to more traditional natural fibers used in earlier hemostatic constructs. Illustrations of such typical disclosures are summarized below.
EP Application No. 99933226.5 described a local absorbent hemostatic material coating the surface of fibers composed of material having biocompatibility and which can be degraded and absorbed in the living body, with extracted collagen. The hemostatic material having biocompatibility and which can be degraded and absorbed in the living body was selected from the group consisting of polyglycolic acid, polylactic acid, copolymer of glycolic acid and lactic acid, polydioxanone, copolymer of glycolic acid and trimethylene carbonate, mixtures of polyglycolic acid and polylactic acid, and oxycellulose.
U.S. Pat. No. 5,679,372 described an absorbable spun, cotton-like topical hemostat containing fibers entangled with each other and being made of atelcollagen obtained by reconstituting solubilized collagen. Each of the fibers has a diameter of 10 to 70 μM and a length of 3 to 70 mm. At least a part of the collagen molecules constituting the fibers are crosslinked by heat at a temperature of 50° to 200° C. The hemostat is swellable upon contact with blood. In use, the hemostat readily adapts to the shape of the hemorrhagic site, has an adhesiveness to a bleeding surface and provides an effective suppression of hemorrhage.
Growing interest in absorbable polymers in the form of scaffolds for tissue engineering has revived interest in electrospinning of synthetic polymers and, in particular, absorbable ones to produce nanofibers and microfibers with exceptionally high surface area. And a logical extension of this integrated know-how was the development of absorbable nanofibers and microfibers for use in hemostatic constructs. An outline of a generic electrospinning process and illustrations of key developments are summarized below.
Electrostatic spinning (or simply electrospinning, ES) is the manufacturing technique most often associated with the production of polymeric nanofibers. In this technique, a polymer is dissolved in a solvent or melted and placed in a glass pipette tube, sealed at one end with a small opening in a necked down portion at the other end. A high voltage potential up to 50 kv is then applied between the polymer solution and a collector near the open end of the pipette. This process can produce nanofibers with diameters as low as 50 nanometers, although the collected web usually contains fibers with varying diameters from 30 nm to over one micron. The production rate of this process is often measured in grams per hour per spin hole or nozzle, and the fiber strength (grams/denier) is thought to be very low, but is difficult to measure.
For tissue engineering, it has been recognized that components of biocompatible scaffolds or matrices or nanometer or micrometer diameter fibers provide favorable environments for cell adhesion, cell proliferation, and directed cellular growth. Fibrous and fibrillar organic and inorganic materials of nanometer or micrometer diameter can be constructed into non-woven, three-dimensional matrices conducive to cell seeding and proliferation. These three-dimensional scaffolds or matrices can then be fabricated into appropriate shapes to stimulate hierarchical micro- and macro-geometry of tissue and or organs to be repaired or replaced. Accordingly, the inventor of U.S. Pat. No. 6,689,166 argued that with new developments in wound healing and tissue engineering, it would be of great advantage to substitute absorbable/biodegradable constructs that are commonly used in medical applications comprised mostly of fibers having diameters that exceed 10 μM with those made of electrospun fibers having smaller diameters. Examples of these applications can include surgical reconstruction and tissue replacement procedures associated with trauma, pathological degradation, or congenital deformity of tissues. Reconstructive surgery is based upon the principle of replacing defective tissues with viable, functioning alternatives. In skeletal applications, surgeons have historically used bone grafts. The two main types of bone grafts currently used are autografts and allografts. Both types of bone grafts have several limitations and synthetic alternatives will be most desirable. U.S. Pat. No. 6,689,166 also describes a tissue engineering device comprising a matrix of biocompatible non-woven nanofibrils comprising a non-degradable polymer selected from the group consisting of polyethylenes and polyurethanes or a degradable polymer selected from the group consisting of poly(lactic acid-glycolic acid), and poly(lactic acid), poly(glycolic acid), poly (glaxanone), poly(orthoesters), poly(pyrolic acid), and poly(phosphazenes). It was further disclosed that the tissue engineering devices comprise absorbable organic polymers in the form of nanometer fibers that are produced by electrostatic spinning and an inorganic component made of calcium phosphate-based ceramic material.
U.S. Pat. No. 6,685,956 described biodegradable and/or bioabsorbable fibrous articles and methods for using the articles in medical applications. The biodegradable and/or bioabsorbable fibrous articles, which are formed by electrospinning fibers of biodegradable and/or bioabsorbable fiberizable material, comprise a composite (or asymmetric composite) of different biodegradable and/or bioabsorbable fibers. The patent further discloses (1) that the biodegradable and/or bioabsorbable fibrous articles are formed by electrospinning fibers of biodegradable and/or bioabsorbable fiberizable material, in which the article contains a composite of different biodegradable and/or bioabsorbable fibers; (2) that preferably, the biodegradable and/or bioabsorbable fiberizable material is a biodegradable and/or bioabsorbable polymer—the biodegradable and/or bioabsorbable polymer preferably contains a monomer selected from the group consisting of a glycolide, lactide, dioxanone, caprolactone, trimethylene carbonate, ethylene glycol, and lysine; and (3) that fibrous articles formed by electrospinning different fibers of different materials, in which the article contains a composite of different fibers containing fibers of at least one biodegradable material and fibers of at least one non-biodegradable material (preferably, the compositite of different fibers contains submicron diameter fibers—the composite can be an asymmetric composite).
Electrospinning is a useful process to produce polymeric fibers in the average diameter range of 100 nm to about 5 μM, which is associated with a number of attributes. These fibers (1) possess a high aspect ratio that leads to a large specific surface; and (2) have been suggested to find applications ranging from optical and chemo sensor materials, nanocomposite materials, nanofibers with specific surface chemistry to tissue scaffolds, wound dressings, drug delivery systems, filtration, and protective clothing. The effects of several process parameters, such as the applied electric field strength, flow rate, concentration, distance between the capillary and the target, have been explored in great detail for different polymer materials. Most of the systems that have been investigated to date have utilized electrospinning from a single solution or melt. Going further beyond single nanofiber and microfiber constructs, a new direction of nano-/microfibrous blends has been pursued by a number of investigators and inventors. In a successful attempt to produce nanofiber/microfiber composites with two distinctly separated fibers, a number of investigators [Polymer Preprints, 44(2), 82 (2003)] were able to design an electrospinning device where two polymer solutions have been electrospun simultaneously in a side-by-side fashion. This allows having a bicomponent system that will have properties from each of the polymeric components, e.g., one of the polymers could contribute to the mechanical strength while the other could enhance the wettability of the resulting non-woven web. The wettability of the electrospun mat can also be controlled by varying its porosity. By a systematic change of one of the process parameters (say the distance between the capillary-end and the grounded target or the flow rate), while keeping the other constant, the porosity of the electrospun mat can be altered. This study also described a new bicomponent electrospinning device and presented results of poly(vinyl chloride)/segmented polyurethane (PVC/Estane) and poly(vinyl chloride)/poly(vinylidiene fluoride) ((PVC/PVDF) bicomponent fibers.
Meanwhile, U.S. application Ser. No. 10/267,823 described medical constructs made of microfibrous blends of absorbable and water-soluble polymers. And this application deals with a biodegradable, absorbent microfiber comprising a substantially homogeneous mixture of at least one hydrophilic polymer and at least one biodegradable polymer. The absorbent fibers can be prepared by an electro hydrodynamic spinning of a substantially homogeneous polymer mixture and used as medical dressing for burns and wounds, cavity dressings, drug delivery patches, face masks, implants, drug carriers that comprise at least one microfiber electrospun from a polymer mixture. The dressings can have variable water vapor penetration characteristics and variable biodegradation times. Some embodiments of the invention provide dressings, implants, dermatological compatible compositions and drug carrier compositions which include totally biodegradable non-gel materials having water, blood, and other biological liquids absorption ability and possessing biological active properties like haemostatic and wound healing acceleration ability, which are irreversible, retain their contour and shape when wet, and do not exhibit any swelling. Additional embodiments provide totally biodegradable microfiber absorbents on the base of blends of synthetic biodegradable polyesters and poly(N-vinyl) lactams. These materials can be used in a variety of products such as cavity dressings, drug delivery patches, face masks, implants, drug carriers, wound and burn dressings with predicable biodegradation times and controlled absorption of biological liquids including blood, and with variable vapor penetration and controlled drug release for wounds and burns.
Although the prior art discussed in the preceding paragraphs deals with several aspects of hemostasis and means to achieve it, including the use of non-woven compositions made of nano/microfibers, it fails to describe novel means or compositions needed to modulate the hemostatic process at different biological sites with different requirements for optimum hemostasis that insures maximized functional performance. This provided the incentive to explore the subject of this invention. Accordingly, this invention deals with (1) the technology of electrostatic spinning to produce nanometer and micrometer diameter fibers with exceptionally high surface area for maximized effect on blood flow to initiate the clotting cascade through contact activation; (2) produce inherently compliant and elastic components of the fibrous construct through the use of segmented crystalline copolymers having triaxial or polyaxial chain configuration (i.e., three or many chain branches extending outward from a central atom); (3) controlling the composition of electrostatically spun fiber precursors to provide constructs with controlled solubility and biodegradability and hence, modulated short- and long-term retention of mass and biologically relevant properties; (4) controlling the surface charge of the electrostatically spun fibers to physicochemically modulate the hemostasis at will; and (5) judicious incorporation of bioactive agents to prevent infection, pain, and/or promote desirable biological events.